Wind energy turbines generate their electrical power from the wind. This power generation occurs when the wind is within certain operational speed limits. Because atmospheric conditions create wind, in icing conditions wind energy turbines could operate at one hundred percent of their operational limits, if it were not for ice accumulation. As such, the turbine's efficiency to produce electrical energy is compromised when ice builds up on the turbine blades.
Unlike other forms of energy production, wind energy is free from waste, provides little environmental impact, exhibits low maintenance costs and offers scaleable capital expenditures. This scaling of wind energy production occurs in the number of units and over a number of different sized units. As few as one device can be installed for local production or as many as hundreds, or thousands installed, creating what is known as a wind farm. Moreover, it has been observed that certain localities offer consistent prevailing winds, making energy production and its management even more cost effective. Unfortunately, while some proportion of existing farm sites are hindered by icing, further candidate sites for wind farms are neglected due to their adverse icing environment. Thus, the ability for an area to sustain winds is not enough. It must be relatively ice free.
Icing has several negative characteristics that affect wind energy production. Blades for wind generation are designed and constructed to the highest level of expertise to provide the maximum transfer efficiency of wind to rotational energy and finally electrical energy. When icing conditions occur, ice accumulates on the blade surfaces, changing the aerodynamic profile, and reducing its efficiency. Furthermore, different forms of ice can have different affects. Some types of ice severely increase aerodynamic drag while adding little weight. This form of accumulation results in less production efficiency as the accumulation continues. At some point, the quantity of ice or its cumulative effect can stall the blades altogether ceasing rotation and electrical energy production. Other forms of icing add weight with less aerodynamic penalty. Increases in weight provide more strain on equipment, thus high maintenance costs. With accumulation of ice, dangers exist when the ice sheds from the rotating blades in an uncontrolled manner. Shedding ice can create hazardous asymmetric loads on all or part of the equipment and destroy the structure. Additionally, shedding of larger pieces can destroy neighboring structures.
During seasonal periods, atmospheric ice conditions exist that render wind energy turbines inefficient and dangerous to operate. Icing conditions are an atmospheric phenomenon that produce airborne water in near freezing, frozen and supercooled-unfrozen forms. Combating ice accumulation can be accomplished by electrothermal means. Until now, electrothermal heating and deicing/anti-icing of wind energy turbine blades has been accomplished by electrical heating elements manufactured from various types of wire and foils. This method of heating the turbine blades is ineffective, suffering from thermal and mechanical fatigue, thereby limiting their life-span. Thus, new and improved methods for heating wind energy turbine blades are needed, which would generate even distribution of heat for more efficiently removing ice accretions and which would improve the fatigue life over wire and foil heaters so as to increase the operating environmental window and life expectancy of the blades.
Conductive fabrics have been used in deicing and anti-icing aerospace structures. For example, U.S. Pat. No. 5,344,696 to Hastings et al. discloses an integrally bonded laminate that is used to thermally control a surface of an aircraft to which the laminate is bonded. The patent describes that the use of fabrics have numerous advantages over prior methods for deicing and heating airplane wings; for example, the conductive fiber is of low weight, and or permits nominal intrusion in terms of aerodynamics; it is easy to handle compared to wire and foil, and most notably, it allows the even distribution of heat. These factors contribute to a more efficient use of energy. Deicing and anti-icing aircraft applications necessitate an extreme in terms of product requirements. Because aircraft operate on very limited electrical resources and extreme atmospheric conditions, a system must be efficient as well as robust to provide protection. A variety of heater elements exist in the prior art.
U.S. Pat. No. 4,534,886, to Kraus et al., discloses an electrically conductive web composed of a non-woven sheet of conductive fibers and non-conductive fibers. The sheet is saturated with a dispersion containing conductive particles and is then dried. The Kraus et al. heater element is used primarily in heating pads.
International Application No. PCT/US94/13504 (Publication No. WO95/15670) discloses an electrically conductive composite heating assembly. The assembly has an electrically conductive non-woven fiber layer laminated between layers of fiberglass and other dielectric material. The assembly further has an abrasion resistant outer layer. The heater element is used on aerospace structures as an ice protection system to withstand the repeated mechanical stress and thermal cycles encountered in extremely harsh aerospace environments.
None of the prior art heater elements, however, have been applied to heat and deice the surface of wind energy turbine blades.